Aluminium is often produced by means of an electrolysis process using one or more aluminium production electrolytic cells. Such electrolytic cells typically comprise a bath for containing bath contents comprising fluoride containing minerals on top of molten aluminium. The bath contents are in contact with cathode electrode blocks and anode electrode blocks. Aluminium oxide is supplied on regular intervals to the bath via openings at several positions along the center of the cell and between rows of anodes.
The electrolytic reaction taking place in the electrolytic cells generates a hot effluent gas that comprises gaseous components that are undesirable in the atmosphere, including hydrogen fluoride, sulphur dioxide, and the like. The process also generates fine dust. The undesirable gaseous components and dust must be disposed of in an environmentally conscientious manner; hence, the raw gas is collected at the electrolytic cells and transported to a gas cleaning unit, where any undesirable components are preferably removed as efficiently as possible. Dust and gaseous components such as hydrogen fluoride may be returned to the aluminium production cells, where they may be of benefit to the production process.
A typical gas cleaning unit may comprise a dry scrubber and a dust filter, e.g. a fabric filter that may be of the bag filter type. A raw gas collection system is typically arranged for collecting raw gas from a plurality of electrolytic cells, and transporting the raw gas to the cleaning unit. A consideration of such systems is that energy-consuming fans are often necessary in flue gas treatment systems to actively draw the raw gas from the electrolytic cells and through the gas cleaning unit. Such is the case since the raw gas collection ducts and the gas cleaning unit may introduce flow resistance in the flue gas collection and cleaning systems.
WO 03/001106 discloses a duct system for transporting raw process gas from a plurality of individually located aluminium smelting pots to a central gas filtering plant. WO 03/001106 addresses the energy consumption of gas transport by providing each branch duct, where it opens into the common collection duct, with a narrowing outlet section that is parallel to the common collection duct gas flow, so as to discharge the branch gas flow into the common collection duct at a speed that is higher than the speed of a gas flow in the common collection duct. Thereby, less energy is required for transporting the gas. There is however a need for even further reducing the amount of energy required for transporting raw gas.